The invention relates to a method of manufacturing a metal matrix which can suitably be used for the manufacture of optically readable synthetic resin information carriers. The manufacture of synthetic resin information carriers can for example, be carried out by means of a compression moulding or injection moulding process. In these processes, a heated, liquid synthetic resin such as polycarbonate or polymethylmethacrylate is forced into a mould at a raised pressure, which mould comprises one or two of the above-mentioned metal matrices. After the synthetic resin has cooled and solidified, the information carrier obtained such as a Compact Disc.TM. or Laser Vision Disc.TM. is removed from the mould. One or both surfaces of the synthetic resin information carrier is or are provided with an optically readable structure which is a copy of the optical structure of the matrix or matrices.
In the customary method of manufacturing a metal matrix, a master disc is used which consists of a glass plate which is provided on one side with a layer of a positive photoresist. On the side of the photoresist layer, the master disc is exposed to laser light which is modulated in conformity with the information to be recorded. In a subsequent developing step the exposed parts of the photoresist layer are removed so that information bits, hereinafter also termed effects, are formed. Subsequently, a metal layer such as an Ag layer is applied to the developed photoresist layer by means of an electroless deposition process such as a vapour-deposition process, a sputtering process or a chemical plating process. A further metal layer such as an Ni layer is applied to the metal layer by electrodeposition. After the master disc has been removed a metal matrix (father disc) is obtained whose optical structure is a copy of the structure of the exposed and developed photoresist layer. Additional metal copies of the father disc can be manufactured by means of electrodeposition. These copies are called mother and son matrices. The latter matrices are often employed in the above-mentioned manufacture of synthetic resin information carriers.
Since the master disc is a product from which a number of matrices and, subsequently, many thousands of synthetic resin information carriers are derived, it has to meet very high quality requirements. The above-mentioned effects are arranged in a spiral-shaped track. The width of the effects amounts to approximately 0.5 .mu.m. The distance between the turns of the track, i.e. the track pitch, is 1.6 .mu.m. The length of the effects varies from approximately 0.9 to 3.6 .mu.m. The length of the effects determines the information recorded. On using an EFM (eight out of fourteen modulation) signal, the length of the effects must vary, as stated above, in discrete steps of 0.3 .mu.m between the smallest (0.9) and the largest (3.6) length dimension. This means that it must be possible to provide and, moreover, detect optically and distinguish effects having length dimensions of 0.9; 1.2; 1.5; 1.8; 2.1; 2.4; 2.7; 3.0; 3.3; and 3.6 .mu.m.
Owing to this, high demands are imposed on the mechanical accuracy with which the information track is provided. This requires the use of expensive equipment comprising laser measuring systems and air cushion supporting means.